I recently acquired a HONEYWELL FUTURAMIC STROBONAR flash, mainly because I thought it looked cool and would be more in keeping with my Century Graphic than the eminently more sensible Metz I already have (flashbulbs are getting expensive!!!).

It seems to work OK but it looks to be giving out a couple of stops less light than I would expect from the calculator on the back.

My question is, how do old flashes deterioate? Does the strength of the flash get progressively weaker? Is it able to hold less and less charge? or does it just take longer to reach a full charge? If one of the first 2, I guess there isn't much more I can do as I am sure repairing it would be pretty expensive. If the last option, how long is it likely to take to recharge (there is no ready light!!!).

Not a direct answer to your questions but the Owner's Manual of my Honeywell Strobonar 770 says to rejuvenate the capacitor "flash the unit four or five times, 25 seconds apart. The capacitor will then be rejuvenated."
There might be a ready light above the index mark for setting the film speed on your unit. The one on my 770 is not noticable in daylight unless you know just where to look. It's quite dim when lit and is totally invisible when not lit.

On any old flash units it is the capacitors that go bad or deteriorate. Electrolytics are big cans of aluminum foil and salt water(electrolyte). The mixture eventually goes south. Luckily capacitors are cheap and MUCH smaller than they used to be. Get one with a voltage rating equal to or higher than your present unit. Get a capacitance rating near what you have. Use a 470uf for a 500uf for example. The capacitance values are not real critical. Trigger caps ALWAYS go bad that is why old flash heads like Stroboflash heads wont file. Two bucks worth of caps and you are back in business.

Oops! Forgot to tell you that strobe capacitors store a LOT of energy and must be fully dicharged prior to messing with them. Sometimes enough energy to kill if the strobe cap is big enough. Get a 750 ohm 10 watt power resistor. Solder 1/2 of a volt meter test lead to each lead of the resistor. Wrap the conections with electrical tape. Discharge the caps by touching the leads across the terminals of the capacitor. This works a lot better than the screwdriver approach AND does not melt the screwdriver or scare the pants off of your wife in the next room when she hears the explosion.

Don't event think of working on Studio flash units like unless you know what you are doing. It could be your last.